Is there a stack? predicate afterwards?
On Sep 4, 2009, at 2:00 PM, Robby Findler wrote:
> I've added #:exists to provide/contract, as a way to hide information
> ala "type t" declarations on ML signatures.
>> See the contracts section in the Guide for a worked example and a
> discussion of a gotcha. The short version is that you can now write
> things like this:
>> (provide/contract
> #:exists stack
> [new stack]
> [push (-> int stack stack)]
> [pop (-> (and/c stack non-empty?) int)]
> [non-empty? (-> stack boolean?)])
>> and have the contract system enforce data abstraction, even if your
> stack operations are simply these:
>> (define new '())
> (define push cons)
> (define pop car)
> (define non-empty? pair?)
>> That is, clients of your module will not be able to treat your stacks
> as if they were lists, even though they really are lists.
>> Robby
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